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  <script>
    // 基本数据类型: number string boolean undefined null

    // boolean 布尔值  true， false]
    console.log( true  + 100 )  // 101
    console.log( false + 99) // 99

    var age = undefined
    console.log( age )

    var obj = null
    console.log( obj )
  </script>
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